A closed vessel which contains functional parts, such as a compressor housing, must be sufficiently sealed and have a high sealing performance. However, conventional compressor housings have valved apertures, secured by a bolt or a blind plug, for enclosing refrigerant. Refrigerant passes through the valve opening before the opening is closed. However, refrigerant or oil leakage from the compressor housing through the apertures often occurs even though the apertures are closed by bolts or plugs.
Sealing mechanisms to securely seal apertures are well known. Such sealing mechanisms include washers, made of relatively soft metals such as aluminum alloy or copper, and bolts made of a relatively hard metal such as iron. However, when the bolt is screwed to the outer surface of the compressor housing, the washer rotates together with the head portion of the bolt. Therefore, the surfaces of the washer and the aperture rub against each other. These surfaces become rough and the sealing performance between these surfaces is reduced.
For example, when the opening of the aperture of a compressor housing, which is made of soft aluminum alloy, rubs against the surface of a softer copper washer during rotation of the bolt, aluminum alloy is scraped from the aperture opening by the washer and adheres to the surface of the washer. This aluminum alloy scrapes additional aluminum alloy from the opening of the aperture. This decreases the sealing performance of the compressor housing, and permits refrigerant fluid or oil to leak out. As a result, the compressor may be damaged. Furthermore, replacement of the old bolt with a new bolt to improve the seal is ineffectual because of the worn opening, and maintenance becomes difficult.